In the field considered here, pneumatic grippers of varying configuration, size and gripping or clamping power are already known and available. Also known among these various types of pneumatic grippers are parallel grippers of the so-called long-stroke or wide-opening type, which essentially comprise a gripper body, two pneumatic control pistons operating within said body, movable in opposite directions with respect to each other and each having a coaxial pushing/pulling stem, two guide columns sliding within the gripper body in parallel to the control pistons and dragged by the same control pistons, and at least two jaws, or fingers, movable by the pistons between an active position for gripping a work-piece and an inactive position for releasing the work-piece.
In one embodiment, the pneumatic control pistons are arranged and operate inside parallel chambers formed in the gripper body, while the guide columns are two in number and are driven within longitudinal seats also formed in the gripper body, along the sides the chambers for the pistons. In another embodiment, the pneumatic pistons are aligned and operate inside the same chamber, with respective stems directed in opposite directions and parallel to guide columns which are also driven longitudinally within the gripper body along the sides of said chamber, as disclosed, for example, by documents JP 62-132005 and U.S. Pat. No. 6,309,003.
In any case, the stem of a first piston and a guide column are directed and extend towards the outside of the gripper body through one of its ends, while the stem of the second piston and the other guide column are directed and extend towards the outside of the opposite end of the gripper body.
One of the gripping jaws or fingers of the gripper is attached both to the stem of a piston and to a column directed in the same direction, and the other jaw is attached both to the stem of the other piston and to the column which are directed in the opposite direction. Usually, each jaw is attached to the associated stem and column not directly, but through a support base.
The jaws are displaced into either an active position or a passive position, depending on the displacement of the pistons in opposite directions.
These parallel grippers can be used to grip a work-piece from the outside or inside thereof, if the work-piece has a cavity. In order to grip a work-piece from the outside, the jaws are brought into the active position when the pistons are displaced so to move the jaws from a position where they are far from each other to a position where they are close to each other. On the other hand, in order to grip a work-piece on the inside, the jaws are brought into the active position when the pistons are displaced so as to move the jaws from a position where they are close to each other to a position where they are far from each other.
The movements of the jaws from a position to the other can be synchronized by a rack and pinion mechanism associated with the control pistons.
However, such pneumatic grippers are not without drawbacks and disadvantages.
Clearly, the presence of the longitudinal seats for the guide columns, situated parallel with to the sides of the control pistons within the same gripper body, limits the space available for forming the piston chamber(s). Consequently, the chamber or each chamber has a bore which is limited in size or, in any case, relatively small compared to the transverse dimensions of the gripper body, so that each piston will have a diameter which is equally small, resulting in a limited clamping power.
Therefore, under these conditions, in order to achieve a higher clamping power, it is necessary to employ grippers larger in size which disadvantageously occupy a larger volume and are also heavier as well as difficult to be manufactured.
Moreover, each variation in length of the closing and opening strokes of the gripper in relation to the work-pieces to be gripped, as required, may be performed either by replacing the jaws with other jaws having thicknesses which are different in each case, or by providing them with spacers.
The stems of the pistons and the guide columns may be provided with seals, but according to the solutions provided hitherto by the prior art, the wear of these seals results in the penetration of dirt and/or fluids into the gripper body, in which case the gripper has to operate in wet environments which can reduce the efficiency and working life of the tool.